This invention relates to control systems for motor vehicles and more particularly to an electronic control system for a motor vehicle transmission.
Motor vehicles since their inception have required some manner of gear change mechanism to satisfy the varying torque and speed requirements encountered during the typical duty cycle of a motor vehicle. For many years these gear change mechanisms were manual in the sense that they required an operator input from a shift lever or the like to effect each desired gear change ratio. More recently, so-called "automatic" transmissions have become popular in which much of the shifting is done without operator input in response to sensed speed and throttle opening parameters. These automatic transmissions typically include a mode select member on the transmission housing movable between a plurality of selected positions corresponding to a respective plurality of shift modes within the transmission. The mode select member is moved between its several shift positions by a cable or linkage mechanism extending from the mode select member to as suitable gear selector lever located in the passenger compartment of the vehicle. Various proposals have been made in the past to eliminate the mechanical interconnection between the driver operated lever and the mode select member and provide instead an electric shift apparatus in which electrical signals generated by a suitable action on the part of the driver are transmitted electrically to some manner of power means arranged to move the mode select member. Electric shift systems of this general type are shown, for example, in U.S. Pat. Nos. 4,790,204; 4,817,471; 4,841,793 and 4,843,901, all assigned to the assignee of the present application.
It is important in such electric shift systems that some means be provided to continuously display the present shift position of the transmission to the driver and it is further important to ensure that the means for displaying the present transmission shift position are working properly.